The formula with 7.9 substituted
miles = 7.9 ÷ 1.609344 = 4.909
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Kilometres to Miles Converter explains where each comes from and gives a longer table.
At a walking pace of 5 km/h, 7.9 km takes about 95 minutes. At an easy running pace of 10 km/h it takes about 47. In metres it is 7,900.
What 7.9 kilometres means in practice
7.9 km is 4.91 miles, five miles minus the rounding: five miles is 8.047 km. It is roughly what a GPS reports for a five mile course that cut a corner or two. Just under twenty laps of a track.